In detonating a plurality of blasting charges, it is often required that the timing of such detonations be controlled precisely. This is true, for example, in quarry blasting, where sequential delays between charges must be controlled within milliseconds. In order to control such timing of charges, transmission tubes are deployed from a central initiating point to send a signal to detonate individual blasting charges. Normally, these transmission tubes consist of one or more main trunk lines connected to a plurality of down lines.
The timing of the detonations is normally controlled by using a preselected length of signal transmission tube, such as a shock tube or deflagrating tube, connected to a detonator consisting of a housing which encloses a delay train and an explosive output charge. Where additional delay time is required, a delay unit may be inserted intermediate the transmission tube ends, as disclosed in U.S. Pat. No. 4,742,773.
The transmission tube may be of the type disclosed in U.S. Pat. No. 3,590,739, sold under the trademark "Nonel", and sometimes referred to as "shock tube". As used herein, the term "signal transmission tube" refers to any detonating or deflagrating signal transmission tube or line including a flexible hollow tube, which can carry a detonating or deflagrating signal along its interior, which signal does not destroy the tube. An alternative transmission device may consist of detonating cords and the like.
The term "signal" when used in connection with the aforementioned transmission tube is intended to refer to both the detonating shock wave or deflagrating flame front which is transmitted along the interior of the tube by combustion of the reactive substances contained therein. The detonator is activated by first initiating the transmission tube, which transmits a signal by propagating the temperature/pressure reaction down its length and into the detonator. The incoming signal ignites the delay train which contains a pyrotechnic composition that burns at a controlled rate in a linear fashion toward the opposite end, which is in contact with an explosive output charge. Where a delay train is used in a transmission tube delay unit, the opposite end of the delay train is in contact with a second section of transmission tube. The signal from the second section of transmission tube can then be used to ignite a further delay train in a detonator.
The rate at which the pyrotechnic reacts and the length of the delay train provides the designed functioning time to which the delay train was made. The rate at which the pyrotechnic burns is a function of the pyrotechnic chemical composition, and the temperature and pressure at which the composition burns.
Delay trains may be provided to operate at various functioning times by proper selection of delay train length and chemical composition. However, the reaction pressure from a transmission tube may vary, causing changes in the functioning time of the delay train. An increased pressure from the transmission tube causes an increased rate of burning, thereby resulting in a shorter than desired functioning time. Similarly, a decreased pressure from the transmission tube causes a decreased rate of burning, thereby resulting in a longer than desired functioning time.
Another problem associated with conventional delay trains is that after the transmission tube ignites the pyrotechnic of the delay train, the interior of the detonator or delay unit housing, being a closed system, becomes highly pressurized. This high pressure condition may cause rupture or ejection of the transmission tube from the housing, causing a rapid depressurization which may result in the separation of the reacting pyrotechnic from the unreacted pyrotechnic, thereby resulting in propagation failure. Such a depressurization may be so violent that the reacting pyrotechnic is physically sucked out of the delay train.
A high pressure pulse from the transmission tube may also cause variations in the effective length of the delay train. The pressure pulse may blow out a portion of the delay train pyrotechnic, or cause changes in the density of the pyrotechnic, which could alter the rate of ignition and the depth of ignition into the pyrotechnic column, thereby resulting in variations in the desired functioning time.
Variations in the functioning time of detonators and delay units in actual blasting conditions may result in out-of-sequence bore hole detonations, thereby causing increased ground vibrations and fly rock, and reduced control of fragmentation. Failure of a detonator in a blast pattern may cause the bore hole explosive to remain uninitiated and become buried and mixed with the fragmented burden. This creates a significant safety problem during digging and removal of burden which contains live explosive and a failed but still live detonator.
It is therefore an object of the present invention to provide an improved signal delay assembly for use with a detonator or a signal transmission tube delay unit.
It is another object of the present invention to provide control of the rate that pressure is applied to the delay train pyrotechnic.
It is a further object of the present invention to provide a delay assembly with a functioning time which can be accurately predicted.
It is another object of the present invention to provide a delay assembly which has improved reliability.
It is a further object of the present invention to provide a delay assembly which securely retains the reacting delay train pyrotechnic.
Other objects will be in part obvious and in part pointed out in more detail hereinafter.
A better understanding of the objects, advantages, features, properties and relations of the invention will be obtained from the following description and accompanying drawings which set forth certain illustrative embodiments and are indicative of the various ways in which the principles of the invention are employed.